The invention relates to a method of improving the adherence of metallic conductive lines on polyimide layers in integrated circuits.
In the production of integrated circuits cover layers are frequently used which have openings for different purposes. The cover layers are provided to protect the pn junctions of integrated circuits against outside influence, and can consist of either inorganic or of organic insulating material. Examples of inorganic insulation materials for making cover layers are SiO.sub.2, Al.sub.2 O.sub.3, amorphous silicon, and Si.sub.3 N.sub.4. It is, for instance, known to provide in integrated semiconductor components a glass layer as cover layer over an underlying passivation layer of SiO.sub.2, and a conductive line pattern on the SiO.sub.2 layer which serves as electrical circuit connection. For making external connections to the conductive line pattern, openings are etched into this glass layer. The disadvantage of these glass layers is that their application is very complex. Furthermore, these layers may be difficult to to etch. Examples of some insulation materials for forming cover layers are polyimides as set forth in GE-AS 1 764 977, GE-OS 1 955 730, polyhydantoins as set forth in 2 322 347 and GE-OS 2 537 330, and a photoresist that is metal-free and cured by heat treatment, as set forth in GE-OS 2 401 613.
It is also known to use in double layer metallization systems, materials of the above named type for making the insulating intermediate layers. For example, a quartz layer was deposited by means of cathode sputtering metallization pattern of a double layer metallization, and after the via holes had been made a metal film was deposited thereon for making the second metallization pattern. A disadvantage of intermediate layers of inorganic materials is, as mentioned above already, that their application is quite complex and that they are highly sensitive to mechanical stress.
Insulating intermediate layers of organic material, for instance of polyimide resin, are much less complicated in their application than quartz layers. These layers also have a much higher layer perfection than that of insulating quartz layers. It is taught in U.S. Pat. No. 3,179,634 that layers of polyamido carboxylic acid adhere better to aluminum substrates after having been dried at 120.degree. C. during a period of 15 minutes, but it has been found that in the etching of metallization patterns in aluminum films vapor-deposited on polyimide substrates that these patterns lift off from the substrate and coil themselves up. Metal films of chromium, copper, gold vapor-deposited on the entire surface show better adherence on the polyimide substrate than those of aluminum, but they have the disadvantage that they corrode and have poor etching properties.
Efforts have been made to improve the adherence of the aluminum layers vapor-deposited on the entire surface of the polyimide substrate. The surface of the polyimide substrate was roughened by means of cathode sputtering so that a satisfactory and sufficient adherence of the aluminum is achieved. There is however the disadvantage that in this process a conductive layer is generated in the polyimide surface which causes very high surface leakage currents. A roughening of the polyimide surface using an alkaline metasilicate solution, a 2.5 percent by weight aqueous tetramethylammonium hydroxide solution, Caro's acid, or N-methylpyrrolidone did not bring success as the surface attack was highly irregular and unreproducible. Another means of roughening the surface consisted of placing the polyimide substrate into an oxygen atmosphere and generate electrical discharges which partly burn the surface. This process can achieve a roughening of the surface of the polyimide substrate. An obstacle to its application is that it is an additional process step. Also there is the risk of introducing electrical charges which would the electrical properties of integrated circuits.